1. Field of the Invention
The present invention relates to a lumbar support device which supports a portion of the back side of an occupant by urging a force thereto.
2. Description of the Prior Art
In Japanese Utility Model Laid-Open Print No. 3-53157 published in 1991 without examination, a conventional lumbar support device is disclosed. The conventional device includes a seat-back having a first upstanding frame and a second upstanding frame which are spaced with respect to each other in the lateral direction; a torsion spring having a first end portion, rotatably supported by the first upstanding frame, and a second end; a supporting plate mounted on the torsion spring, and a driving means disposed between the second end portion of the torsion spring and the second upstanding plate. The supporting plate is expected to be moved toward the back portion of an occupant along the direction of a thickness of the seat-back when the driving means is rotated in order to adjust thickness of the seat-back when the driving means is rotated in order to adjust the supporting force to be applied to the back portion of the occupant.
In the conventional device, the first end portion of the torsion spring is movable relative to the first upstanding frame due to the fact that the former is loosely received in a hole formed in the latter, and the second end of the torsion spring is, as it were, due to the existence of the driving means, fixedly connected to the second upstanding frame. Thus, under such structure, when the supporting plate supports the back portion of the occupant, the degree of deformation of the first end portion of the torsion spring becomes larger than that of the second end portion of the torsion spring. This results in that uneven supporting forces are applied by way of the supporting plate to the backside of the occupant, by which the occupant sometimes may feel discomfort.